Pentre Ifan Geology

There is remarkably little in print relating to the geology of Pentre Ifan -- a pity, since it looks rather interesting. There is some variation in the stones but I would describe most of them (including the 16-tonne capstone) as agglomerates made of volcanic ashes or tuffs -- all belonging to the Fishguard Volcanic Series which outcrops widely in the vicinity.  Some of the rocks have a "flaky" appearance -- but they are not foliated.  The stone surfaces are often "knobbly" and rough, suggestive of quite large nodules or inclusions in a finer matrix of ash.  There are some traces of bedding which might relate to the accumulation of layer after layer during the volcanic eruptions of the time; and here and there fracture planes coincide with the alignments of these layers.  Some stones are quite an attractive dark blue colour where they are exposed as reasonably fresh or broken surfaces.  But such fresh exposures are few and far between -- the vast majority of exposed surfaces are very ancient indeed -- heavily abraded and weathered.  In a few places (as on the underside of the capstone) flattish slabs or flakes of rock have fallen off; my feeling is that such surfaces were face-down in the ground until these stones were erected into the current megalithic monumental setting.